The present invention relates to a data communication apparatus and, more particularly, to a data communication apparatus, such as a facsimile apparatus, which is capable of detecting an interrupt request signal, e.g., a "catch phone" signal without the need for special hardware.
Among the services provided by NTT (Nippon Telegraph & Telephone Company) is a "catch phone" service. This service allows an interrupt call with a station B while the own station is communicating with another station A.
More specifically, when the own station is dialed by the station B while communicating with the station A, a prescribed call signal (interrupt request signal) is sent to the own station from an exchange. If a person at the own station manipulates the hook button in response to this call signal, the station B is connected to the own station, and a prescribed holding signal is sent out to the station A from the exchange. If the person at the own station manipulates the hook button again upon completion of the communication with the station B, the station A is again connected to the own station.
In the following description, the two stations (station A and own station) which are presently communicating with each other will be referred to as "transmitting station" and "receiving station", respectively, and a station B which tries to make interrupt communication will be referred to as "interrupt station".
The specifications of the call signal and the holding signal used in the catch phone service are as follows:
Frequency: 400.+-.10 Hz PA1 Output level: not more than -16 dBm PA1 Tone-on time: 70.+-.10 ms PA1 Tone-off time: 70.+-.10 ms PA1 Period: 4 or 8 sec (generally, 4 sec).
A waveform of these signals is shown in FIG. 7.
To receive the catch phone service in the case of a facsimile apparatus connected to a telephone circuit to communicate of image information, the facsimile apparatus may be provided with a means for detecting the call signal employed in the catch phone service, and circuits may be switched in response to the detection of the call signal.
This type of facsimile apparatus is disclosed, for example, in Japanese Patent Application Laid-Open No. Sho. 62-216584. The disclosed facsimile apparatus is constructed such that, when a call signal of the catch phone service is sent to a receiving station (or transmitting station), an interrupt station is connected to the receiving station upon reception of the call signal. A holding signal is sent out from an exchange to a transmitting station (or receiving station), which was interrupted in the midst of communication, as described above. However, since general facsimile apparatuses do not have a capability of detecting this holding signal, the transmitting station (or receiving station) is placed in a no-signal state, so that this station disconnects the circuit after the lapse of a predetermined time. Therefore, the facsimile apparatus of the above publication is designed such that the receiving station (or transmitting station) automatically dials the transmitting station (or receiving station) with which it previously communicated, upon completion of the interrupt communication.
Japanese Patent Application laid-Open No. Sho. 64-86660 discloses a technique in which a receiving station (or transmitting station) that is in communication with an interrupt station performs a restoring operation on a station previously communicated with, before a predetermined time has elapsed from the start of the interrupt communication. By virtue of this restoring operation, the station of the previous communication will not release the circuit even during the interrupt communication.
In either of the above-described cases, hardware, e.g., a filter, for separating the catch phone call signal from a communication signal, such as image information signal, is needed to detect the call signal. This makes the construction of the data communication apparatus complicated.